It has been observed that, in order to ensure that the mixture of hot air and cold air is homogeneous, such a flap needs to be fitted with air-guiding blades, and that at least one of the blades should be in the form of a block occupying a certain volume rather than being a mere partition or thin wall added to or integrally molded with the flap.
Present practice is to make such a blade as a solid block of plastic or as a block of foam, and to glue the block to the flap. This technique is relatively expensive since it requires a separate mold to be provided for fabricating the block-shaped blade, and then an additional manufacturing step of fixing the block to the flap.
Preferred embodiments of the present invention avoid this drawback.